Blenheim Palace is nice and about 15 mins away by car. In the city centre itself there isn't a great deal. You could go on one of the guided tours of the Colleges. Also worth going up Carfax Tower. It costs about a quid each and the views of the city are fantastic.
What sort of thing did you have in mind?
Going to visit the gf's nan again in oxford, usually we just sit by the lake, go on a boat ride, feed the ducks etc
Anything I can surprise her with? (my gf, not the nan)
In Oxford from thurs to sat am
Burn things. About all that dive is good for.
Get a Litre of Vodka and drink yourself to oblivion.
I lived there for 4 years and its bloody boring and small.